Rotary or centrifugal pumps, in which one or more rotating impellers, each connected to a shaft, pump a liquid from an intake to a discharge, or volute, require the maintenance of very close clearances between rubbing surfaces for their continued volumetric efficiency. Pumps of this kind are frequently used to pump liquids at relatively high temperatures, and are used in applications where it is not possible or feasible to physically inspect the pumps during operation.
For example, the pump structure of nuclear reactor pumps and down hole pumps, cannot be inspected for operating problems, such as deviations in clearances between relatively rotating parts. The rotating parts can include shafts, impellers, and the like. The deviations in clearances can be caused by unbalanced radial forces due to the lack of circumferential symmetry of the various parts, and/or as a result of the elevated temperatures of the liquid being pumped which can cause thermal expansion of the parts.